Frequency measuring device



Jan. 9, 1940. A. c. sTocKR Er AL FREQUENCY MEASURING DEVICE .Filed Jan.8, 1938 2 Sheets-Sheet l xvlm. Rvkmmwk KNS. MNNS wvw N N Rww. .m50 mf@f2 WM m m 5 I w m H A. c. sl'r'oEcKER Er A1. 2,186,182

FREQUENCY MEASURING DEVICE Jan. 9, 1940.l

Filed Jan. 8, 1958 2 sheets-sheet 2 dttorneg Patented Jan. 9v, 19.40

UNITED STATES PATENT rrr-CE `runat;UiiNov MEAsUai-NG DEVICE Arthur C.Stocker and Harold ,J. Schrader,

Haddon Heights, and BenW. Robins, Haddonfield, N. J., 'assignors toRadio"l Corporation of America, a corporation of' Delaware ApplicationJanuary rS, 1938', Serial No. 183,968 5 claims. (o1. 25o-39) i Unknownfrequency by beating-it againstaknownfrequency oscillation and measuringthe result-ant beat frequency. It is also customary for the knownfrequency tol have such relation to l5 they unknown that the lresultantbeat is withinthe audible range, for therebyy the audible beat can beaccurately measured by 'audio frequency standards. 'To do this, it iscustomary to use ay known radio frequency signal vwhich is within 20 l0'kc. of any signal' which is to be measured.

This is usually accomplished by using the harmonies or a i0r kc.multi-vibrator. While suchy a system is'very accurateat low frequencies,itis not entirely satisfactory at ultra high'frequen# 25, cies, due tothe difficulty of determining which harmonic of the multivibrator isbeating against the unknown.l A further difficulty isv met in attemptingto combine vthev very weak higher harmonics of the standard with theunknown signal.

30 .A partial solution toy the problem is to make such' ultra highfrequencymeasurements with the aid-cf an' auxiliary oscillator operatingon a much lower frequency. The auxiliary oscillator is theni adjusted sothat oneof its'harmonics beats with 35 theV unknown signal'. `'lfhefrequencyof' the auxiliary isy then measured, and the result multiplied'by the harmonic used. However, even thismethod is also subject to thelimitation that it is often difficult to know what harmonic is beatingwith m, the unknown.

It is, therefore, an object of our invention to provide a method foraccurately measuring ultra high frequencies by receiving them in 'amultiband superheterodyne receiver in which the local oscillatoroperates in the lowest frequency band.

i High :frequency local oscillations are obtained from casca-dedfrequency doublers so connected that the proper harmonic for each bandis automatically selectedv by the'range switch. A stand- 5m ardfrequency generator is included whereby accurate tuning to theintermediate frequency is made possible and the fundamental of the localoscillator measured. f c.

`Our linvention wil-1v be better understood from the followingdescription, when considered in` the-elements of one cmbodiment'ofourinvention,

connection with the accompanyingI drawings. its scope is indicatedby theappended claims.

Referring tov the drawings: Figure-:l` is av schematic representationshowing certainV units of this device being sowell knownv` to4 thoseskilled in the art that they have not been shownl in detail;

Figure 2V illustrates how the preliminary adjustinent is made; lo

Figure 3 shows the yconditions for reception o the'nnknown signai` andIfor tuning the local oseiilator; and r f Figure 'l shows 'the conditionsfor measuring thev fundamental frequency of the local osciliii' lator.vv

Referring to Fig, i of the drawings, av vesection variable capacitor isshown at i5. The first section Il is connected tothe arm of switchil 4and thereforek tunes the inductor .selectedA by a 2o three-sectionrangeswitch 3. The second sec tion l-9L tunes a local oscillator i3throughout the approximate rangeof Z500-to 5000 kc. The third section 2ltunes the resonant circuit of a' first doubler 2T, which covers afrequency range from 2,5,

5,000to 10,000 kc. The fourth section tunes the second doubler 29 from10,000 to 20,000' ko. The fifth section 25 tunes the third: doubler 59'from 20,000 to 40,000 kc. The multi-unit capacitor I5- is equipped witha precision dial'` H33 which is :iu

accurately caiibrated in termsr of the frequency covered by the localoscillator i3.

The output from Vthe local oscillator i3: is connected to the-firstContact pointl'l to section li of athree-section range switch 3. .f Theoutput 3 5 from the first doubler 2l is connected to: the sec-i`ondcontact point 33.'y The output from the secn ond doubler isvconnected to the third contact point 35, and the output from the thirddoubler is connected to the fourth, iifth' and sixth contact 4U.

points of the switch, as shownat 31, 39 and fil, respectively. Thecontact arm i3 of switch il is connected to the grid' @i5 of a mixertube 4l.

Another section of therange switch 3 is shown at 5; The first grid: E3of the mixer tube 4liand the capacitor Il'` are .connected to the movingarm 'llr of switch 5. Six contact points 65, .0?, S9, '11,73 and l5 areconnected to the ungrounded terminals of six inductors '10, 8i, 03,55,731 and B9', respectively'. The remaining sectionl' ofthe 50,

range switch 3 has its movable'arm 9i connected to the unground'ed* endof the volume control resistor 9. Its sixv contact points 93, 95, 91,9g, |01' and lr03 f connect to points intermediate the ends of therespective inductors 19, 81,83, 85, 81 5 5 and 39. Operation of therange switch, therefore, not only selects an input circuit of asuccessively higher resonant frequency, but also selects a correspondingsuccessively higher harmonic of the local oscillator. For example, inthe first position the range switch connects the input to the tap oninductor 10, connects the inductor 19 to the grid of tube 41, andapplies the fundamental of the local oscillator to the mixer tube 41.vThe inductor 19 and the capacitor I1 are resonant to signals from 1500to 4000 kc. The local oscillator tunes from 2500 to '5000 kc., and sincethe two circuits are simultaneously tuned over their respective rangesby the same control, two signals are combined in the mixer tube 41 toproduce a beat frequency of 1000 kc.

The second position of the range switch 3 selects a reception range offrom 4 to 9 mc., While the switch now supplies the mixer tube witheffective local oscillations from the first doubler 21, covering from 5to 10 mc. The third position selects a range of 9 to 19 mc. in the inputcircuit, and the second doubler 29 now supplies effective localoscillations from 10 to 20 mc. The fourth position resonates the inputcircuit from 19 to 39 mc., while the third doubler is supplyingeffective local oscillations from 20 to 40 mc. On the fifth position,the input covers the 3.9-79 mc. range, but the practical limit offrequency doubling has been reached and so the harmonics in the outputof the third doubler are used to provide effective local oscillationsfrom 40 to 8O mc.

The sixth position co-vers a reception range from y '19 to 159 mc., andonce again harmonics are employed which provide effective localoscillations from 80 to 160 mc. Thus, for each position of the rangeswitch, effective local oscillations are provided 1000 kc. above theresonant frequency of the input circuit for every position of thevariable capacitor I5. Because of the amplification and selection of theproper harmonic, the possibility of error in receiving a signal isgreatly reduced.

The output of the mixer tube 41 is amplied by an intermediate frequencyamplifier 55, tuned to 1000 kc. This is followed by a detector and audioamplifier 51. The output of the audio amplifier is serially connected tothe positive voltage supply through the primary of an audio transformer|01 and a plate current meter |09. A pair of headphones |55 may beconnected across the secondary of the audio transformer |01.

Additional units in this device include a precision crystal oscillatorstandard 49, oscillating at kc. and driving a 10 kc. multivibrator 5|;and an auxiliary oscillator 53. This auxiliary oscillator may include asuitable triode or pentode oscillator which covers a frequency rangefrom 995 to 1005 kc. It is tuned by a variable capacitor which has adial |65 calibrated in frequency deviation in cycles above and below1000 kc. Zero on the dial corresponds to 1000 kc. A small variablecapacitor |53 is connected across I The positive voltage supply to thisauxiliary oscillator is connected through one section of a double switchH3, whereby the oscillator is turned off when the switch is in its firstposition |33, and turned on when in the remaining two positions |35 and|31. The output of said auxiliary oscillator is obtained from a pickupcoil |I5 which is connected to the arm ||1 of the second section ofswitch ||3. A small capacitor ISI of the order of 5 mmfd. is connectedbetween the switch arm ||'I and the third vided.

Contact |2| of the second section of switch ||3. The rst two contacts ofthis second section are not used. Contact |2I is also coupled to theinput of the intermediate frequency amplifier 55 through a similar smallcapacitor I 53.

A second double section switch ||9 is also pro- In the rst section ofthis switch, the arm |23 is connected to the movable arm of the volumecontrol 9. The first and second contact points, |39 and |4I, areconnected to one terminal of a wave trap |21. The remaining terminal ofthe Wave trap is connected to the ungrounded input terminal |25. Thewave trap |21 is resonant at 1000 kc., and its purpose is to prevent thereception of stray signals near the intermediate frequency. The thirdcontact |43 is connected to one terminal of a second wave trap |29. Theremaining terminal of |29 is connected to the output of the crystaloscillator and to the multivibrator. The wave trap |25'is resonant at100 kc. Its purpose is to attenuate the output at the fundamentalfrequency of the crystal oscillator.

ln the second section of switch H3, the movable arm |3| is grounded. Thefirst contact |45 is connected to contact |2| of switch |13, and alsothrough a small capacitor |51 to the output of the crystal oscillator49. The second and third contacts are open, as shown at |41 and |49. Aswitch |51 is providedto permit the multivibrator to be independentlyturned 01T when not in use.

The operation of measuring an unknown signal frequency will now beexplained. It is necessary to make a preliminary adjustment of theauxiliary oscillator frequency. Referring to Fig. l, set the calibrateddial |55 of capacitor at zero. Place switch |I3 in its second position.Turn the volume control 9 to minimum. Turn the multivibrator switch `|51off. Place switch ||3 in its second position so that arm |3| contacts|41.

Fig. 2 shows in simplified form the connections made by the precedingoperations, and it is seen that the tenth harmonic of the crystalstandard will be impressed on the intermediate frequency amplifierthrough capacitors |5| and |53. In addition, a voltage whose frequencyis approximately 1000 kc. will appearv in the intermediate frequencyamplifier through capacitors |53 and I6I from the auxiliary oscillator53. Consequently, a beat note will be heard in the headphones |55.Reduce this beat note to zero beat by means of variable capacitor |59.The meter |09 is very useful for this purpose, for the beat can beobserved by the motion of the meter pointer. This adjustment must bemade very accurately.

The measurement may now be made. It is necessary to reset the switchpositions. Referring again to Fig. 1, select the range on the rangeswitch 3 corresponding to the frequency of the signal to be measured. Ifa modulated signal is to be measured, place switch I|9 in the firstposition. Switch arm |3| then grounds contact |45 and prevents thecrystal oscillator from creating a beat note in the intermediatefrequency amplifier when a signal is received. If an unmodulated signalis to be measured, place switch ||9 in the second position. Sufficientvoltage will then be supplied thev intermediate frequency amplier,through 15| ,and |53, to permit beat reception. Place switch |I3 in itsrst position, thereby stopping the auxiliary oscillator. Advance thevolume control 9 to maximum. Tune sequently a low impedance.v

` oscillator from getting into the amplifier.

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aleeisa capacitor l until theV signal to be measured is received.Ysecond position, it shouldl now be' placed there.

These connections in simplified form are shown in Fig. 3. 'The beat notewhich-is heard is the result of the' combination of the tenth harmonicof the standard frequency oscillator 49 with thedifference frequencyresulting fromthe mixing of the effective local frequency and theunknown signal, in the customary manner of superheterodyne receivers.Tune the local oscillator until an exact zero beat is'obtain'ed;v Themeter |09 is again useful for this purpose. The-local voscillator, orsome harmonicthereof, is now exactly 1000 kc. above the frequency of theunknown signal.y I-t is therefore evident that yif `the localoscillator` frequency can be measured the unknown frequency mayv readilybe determined. It

` is precisely this operation which is accomplishedy by this device. y

As explained above, the local oscillator is operated at frequenciesbetween 2500 and 5000 kc. Reception is accomplished in all but thelowest I range by the use of harmonics strengthened by one or moredoubler stages of amplification Z'l,` v

29y and 59. The actual harmonic used is determ-ined' by the range switchl l, which automatically selects the proper harmonic for the rangerequired. The table belowshows the relation between the reception range,the :oscillator frequency, the harmonic used, and the effectiveoscillator frequency.

^ Funda- Reception mental Harmonic gig? n range oscillator used fr; ucm,f

frequency 'q 'y Meg cyclesy Megacycles Megacycles 1.5- 4.0 2. 55.0lunldamen-f 2.5- 5.0

` to 4 9 2. 5-5.0- 2nd- 5:0- 10 9 19 2.5-5.0 4th- 10 20 19 3Q 2.5-5.0Sth- 20 40 39 79 2. 5-5. 0 16th 40 80 79 1 -159 2. 5-5. 0 32nd- 80 -160The next operation is to measure the funda? mental `frequency of thelocal oscillator. Referring again to Fig. 1, place switch I3 in itsthirdposition so that switch arm lll-contacts |2l, short-circuitscapacitor ll, and connects contact |2| to the pick-up. coil H5. vCoil H5has a vcomparatively lsmall yinductance and Acon- The 1000 kc..voltageappearing across it is applied to the intermediate frequency amplifier,but its low impedance ef,- fectively prevents any voltage from' thecrystal Place switch M9 in its third position so that switch arm |23contacts |43. Turn the multivibrator switch |51 on. The output of the 10kc.`rnultivi brator is now applied to the volume control 9,

' and will appear on the grid 03 of the vmixer tube.

tuned, it beats with one of the kc. harmonics from the multivibrator togive a signal within 5000 cycles of the intermediate frequency. Thissignal beats with the 1000 kc. signal supplied by' the .auxiliaryoscillator to give an audio beat within 0 and 5000 cycles. Capacitor`may If switch H0 is not already in4 the now be tuned until the audiovbeat note is reduced rIfhe fundamental frequency of the localoscillator mayvnow be calculated. The local oscillator frequencynecessarily fallsy betweens two adjacent signals from the multivibratorwhich are 10kc. apart. Reference tothe calibration of the localoscillator on dial |63 will indicate, its approximate frequency, andtherefore the two frequencies which are multiples Lof 10 kc. above andbelow ther local oscillator are known. Thev actualoscillatorfrequencyvisdetermined lby the reading obtainedI from the dial|65. Add this reading to the l0 kostep below the approximate frequency,if the reading is above 1000 kc. Subtract this reading from the nexthigher 10 kc. step if` said reading is below i000 kc. noted that theaccuracy of this measurement is only limited by the accuracy of thecrystal oscillator standard and the audio frequency calibration of thedial |65. l

"If the unknown signal was received in the low frequency range, itsfrequency is equal 'to' the frequency of the local Oscillator, asdetermined above, minus 1000 kc. Inthe .higher frequency bands, it isnecessary tomultiply the local oscillator frequency by a power of 2before subtracting 1000 kc.`

This may be stated in the tabular form shown below, where y fm=measuredfrequency in kc. fo :local oscillator frequency as determined in thevmanner explained above.

Range fm 1.5-4 mc fr1000 4-9 mc 2fn-l000 9-19 mc 4f-1000 19-39 mcSfr-1000 39-79 mc l? -1000 79-159 m0 32h-1000 Wel have thus provided ameans for accurately measuring unknown frequencies in the medium andultra high frequency ranges. Our method is unique in its operation ofthe local oscillator of a multibandl superheterodyne receiver in thevblersas the local oscillator of a superheterodyne receiver. `It beatsthe local oscillator with a crystal oscillator standard and calibratedauxiliary oscillator whereby the fundamental of the local oscillator maybe accurately measured to determine thereby the frequency of an unknownsignal. With a device of the type described, it is possible to attain anaccuracy of 1.005% at any frequency within its range.

.` We claim as our invention:

l. The method of measuring radio frequencies which comprises generatinglocal oscillations which are tunable in a low radio frequency range,amplifying successive harmonics of said local oscillations, resonatingthe unknown `signal to be measured andrat the same time automaticallyselectingthe harmonicl proper for use with said unknown signal, mixingthe unknown signal and the selected harmonic so as to produce anintermediate radio frequency equal to their difference, beating saiddifference frequency against a precision standard frequency, adjustingsaid local oscillations until said difference frequency equals It is tobe 'ill said standard frequency, and measuring the frequency of saidlocal oscillator, thereby determining the frequency of the unknown.

2. In a frequency measuring device, the combination including meansselectively responsive to radio frequency signal currents which are tobe measured. and which fall in any one of a plurality of frequencybands, means for generating local radio frequency oscillations, meansfor tuning said local oscillations over a range of frequencies which isa predetermined amount higher than the range of the lowest frequencyband covered by said responsive means, means for selecting a current'the frequency of which is equal to the fundamental or a known harmonicof said local oscillator, means for combining said signal currents andsaid selected fundamental or harmonic current to obtain a predeterminedinter-- mediate radio frequency current having a frequency equal to thedifference of the frequencies of the combined currents, and means formeasuring the fundamental frequency of said ilocal oscillator todetermine the frequency of said signal currents.

3. In a device for measuring radio frequency signals which comprises amultirange superheterodyne radio receiver whose local oscillatoroperates in a predetermined relation to signals in the lowest frequencyrange and which utilizes harmonics of said local oscillator for thehigher frequency ranges, the method of operation which includesselecting the range of operation of said receiver which corresponds tothe approximate frequency of the signal to be measured and therebyautomatically selecting `the harmonic of said local oscillator which isrequired to produce an aproximate intermediate frequency signal,accurately adjustingl said local oscillator to obtain an exactlypredetermined intermediate frequency signal, and measuring the frequencyof the local oscillator to determine the frequency of the signal.

4. in a frequency measuringy device, the combination which includes aplurality of input circuits responsive to signal voltages ofsuccessively higher frequency ranges; means for selecting one of saidinput circuits; means for tuning said selected circuit over a frequencyrange; a local radio frequency oscillator; means operatively connect--ed to said tuning means for tuning said local oscillator over a range offrequencies which bears a predetermined relation to the range offrequencies covered by the input circuit responsive to the lowest ofsaid successively higher frequency ranges, means for obtainingsuccessive harmonics of said local ocillator; means operativelyconnected to said input circuit selecting means for automaticallyselecting the fundamental or a harmonic of said local oscillator whichbears said predetermined,relation to the input circuit frequency whichis concurrently selected; means for mixing said signal voltage and saidselected fundamental or harmonic of said local oscillator to obtain apredetermined intermediate radio frequency current, and means foraccurately measuring the fundamental frequency of said local oscillator.

5. In a frequency measuring device, the combination including an inputsystem consisting of a plurality of inductors of successively decreasinginductance, a first selector switch for effectively connecting a firstvariable capacitor cross a desired one of said plurality of inductors, athermionic local radio frequency oscillator which is tuned by a secondvariable capacitor operatively connected to said first Variablecapacitor, said local oscillator being a predetermined frequency abovethe resonant frequency of the input circuit covering the lowestfrequency range, a plurality of frequency-doubling amplifiers for0btaining successive harmonics of said local oscillator connected incascade and tuned by capacitors operatively connected to said rst andsecond variable capacitors, a second selector switch operativelyconnected to said first selector switch for selecting a harmonic of saidlocal oscillator which, at every point over its range, is substantiallya predeterminedfrequency above the resonant frequency of the inputcircuit concurrently selected by said first selector switch, athermionic tube having input and output electrodes, means including saidinput circuit for impressing a signal of unknown frequency on an inputelectrode of said tube, means for impressing said selec-ted harmonic onan input electrode of said tube to produce a beat frequency equal to thedifference of said unknown and said selected harmonic frequencies, andmeans for measuring the fundamental frequency of said local oscillator,whereby the frequency of said unknown may be determined.

v ARTHUR C. STOCKER.

HAROLD J. SCHH/ABER. BEN W. ROBINS.

